The present invention relates generally to filtering techniques for use in communication system receivers, and more particularly to analog host signal filtering in a digital audio broadcasting (DAB) system receiver or other type of communication system receiver.
Proposed systems for providing digital audio broadcasting (DAB) in the frequency modulated (FM) radio band are expected to provide near CD-quality audio, data services, and more robust coverage than existing analog FM transmissions. However, until such time as a transition to all-digital DAB can be achieved, broadcasters require an intermediate solution in which the analog and digital signals can be transmitted simultaneously within the same licensed band. Such systems are typically referred to as hybrid, in-band on-channel (HIBOC) DAB systems, and are being developed for both the FM and AM radio bands.
In order to prevent significant distortion in conventional analog FM receivers, the digital signal in a typical FM HIBOC DAB system is, e.g., transmitted in two sidebands, one on either side of the analog FM host signal. Current Federal Communications Commission (FCC) regulations require that the digital signal be transmitted at a power level 25 dB below the host signal. The analog and digital signals thus coexist with very little frequency separation, and in fact may overlap occasionally, depending on the nature of the analog modulation components. For a receiver designed to receive only the digital signal information, it is a relatively straightforward matter to remove most of the host FM signal energy from the received signal through conventional digital filtering techniques. However, this digital filtering can significantly increase the cost and complexity of the receiver. Furthermore, since all digital filtering must be done post-analog-to-digital converter (post-ADC), the ADC must be capable of handling the full dynamic range of the analog and digital signal components. This increases the required performance level of the ADC and thus its cost.
A number of conventional analog filtering techniques provide selective rejection by filtering of an on-channel interferer in a radio communications system. For example, wideband code division multiple access (CDMA) personal communication services (PCS) systems use a bandreject filter at the receiver to selectively remove narrowband interference that spectrally overlaps the desired received signal. This filter may be tunable or fixed in frequency. Although the filter also removes some of the desired signal information, this information is recoverable due to redundancy in the transmitted signal. However, these and other conventional analog filtering techniques are not readily applicable to the above-described HIBOC systems. A need therefore exists for improved analog filtering techniques suitable for use in HIBOC systems, so as to avoid the increased cost and complexity commonly associated with digital filtering.
The present invention provides methods and apparatus for processing received information in digital audio broadcasting (DAB) systems and other communication system applications. In an illustrative embodiment, first and second digital sidebands are transmitted on either side of a frequency modulated (FM) analog host signal in a hybrid in-band on-channel (HIBOC) DAB system. A digital receiver which receives the analog host signal and digital sidebands includes a bandreject filter arranged at least in part in an intermediate frequency (IF) processing stage of the receiver. The received composite signal is filtered in the IF processing stage of the receiver by the bandreject filter in order to substantially attenuate the analog host signal while passing the digital sidebands. The bandreject filter may be, e.g., a unitary bandreject filter implemented at a single point in the IF processing stage of the receiver, or a distributed bandreject filter implemented with multiple sections at a plurality of points in a signal path of the receiver.
In accordance with the invention, the bandreject filter may be arranged in the IF processing stage so as to filter the analog host signal from the composite signal at a point prior to an input of an automatic gain control (AGC) circuit, such that the AGC operating point for the receiver is determined primarily by the digital sidebands. For example, the bandreject filter may be arranged in the IF processing stage between an output of an IF buffer amplifier and an input of the AGC circuit, such that the AGC operating point is determined primarily by the digital sidebands. As another example, the bandreject filter may be arranged in the IF processing stage between an output of an IF bandpass filter and an input of an IF buffer amplifier, such that the AGC point is again determined primarily by the digital sidebands.
Numerous other configurations are also possible. For example, the bandreject filter may alternatively be arranged in the IF processing stage between an output of the IF buffer amplifier and an input of an analog-to-digital conversion (ADC) circuit, with an input of the bandreject filter coupled to a point which is between the output of the IF buffer amplifier and the input of the ADC circuit but after a point which drives an input of the AGC circuit, such that the AGC operating point is determined primarily by the analog host signal. In this case, although the AGC operating point setting is determined primarily by the analog host signal, the digital signal-to-interference ratio and dynamic range utilization of the ADC circuit is improved.
In accordance with another aspect of the invention, the bandreject filter may have a group delay characteristic for a portion of a frequency spectrum associated with the digital sidebands that is selected to equalize a corresponding group delay characteristic for a signal path of the receiver. In other words, the bandreject filter serves not only to provide rejection of analog host signal energy, but also to assist in providing a desired overall group delay performance for the corresponding signal path of the receiver.
One or more characteristics of the bandreject filter may be adjusted based on feedback from a back end digital signal processing element in the receiver. In addition, the bandreject filter may be selectively adjusted, e.g., tuned out of or otherwise electronically removed from the IF processing stage of the receiver, in order to permit reception of the analog host signal.
Advantageously, the invention allows AGC circuitry in the receiver to use the digital sideband signal, rather than the analog host signal, to set the dynamic range of the receiver. Allowing the digital signal to have a greater effect on the setting of the AGC operating point enhances the performance of the digital demodulation process in the receiver, particularly in the presence of multipath reception conditions. In addition, the invention reduces the need for post-ADC host rejection filtering, thereby simplifying the signal processing performance requirements in the receiver back end.
The invention can be applied to other types of digital information, including, for example, data, video and image information. In addition, the invention may be implemented in numerous applications other than FM HIBOC DAB systems, such as Internet and satellite broadcasting systems, systems for simultaneous delivery of audio and data, etc.